The present invention relates to a system for controlling the unwinding of the warp in a loom in which the warp is wound onto at least two beams arranged end to end and rotatable independently of each other.
In weaving, in order to produce pieces of considerable width, for example, three meters wide or wider, looms are often used in which the warp or chain is wound not around a single beam but around twin beams. The preparation of a fairly long warp beam is actually quite problematical. It is, therefore, often preferred to use shorter, twin beams instead of a single beam of exceptional size. Thus, for example, twin beams each 1.5-3 meters long, the latter being a standard size, are currently used for weaving pieces 3-6 meters wide.
If twin warp beams are to be used, there is a problem in correctly controlling the unwinding of the warp in order to ensure the correct and even insertion of the weft threads.
This problem is complex and is not easy to solve if one takes account of the fact that two beams on which the same, nominally identical yarns are wound invariably have different characteristics which may adversely affect the weaving. Thus, even with the use of yarns which nominally have the same characteristics, because of different humidity and temperature conditions during the winding of the warp as well as the inevitably different torsional characteristics of the yarn, it is never certain that, when the two beams are rotated through the same angle, the same length of warp will unwind or that the warp tension will remain uniform.
Moreover, it must be remembered that the tension of the warp depends not only on the rate at which it unwinds from the beams, but also on the elasticity and any longitudinal non-elastic yielding of the yarns used. These characteristics, which are far from being rigorously uniform even in the best yarns, may even vary longitudinally of the same yarn because of its twist and sizing characteristics and may vary from thread to thread in yarn of the same type.
In view of the fact that the warp threads wound onto the twin beams never have exactly the same characteristics, the twin beams are typically mounted in the loom so as to be rotatable independently.
The interposition of a coaxial mechanical differential between the beams has been proposed, in order to compensate for the variations in the tensions of the rows of warp threads unwinding from each beam.
This solution has been found unsatisfactory both because, as a result of play and wear, the differential may bring about insufficiently precise corrections and because, in some situations, the fact that the differential has the characteristic of increasing the speed of one beam as the speed of the other decreases may lead to problems even more serious than those it was intended to solve.
Devices for regulating the unwinding of the warp have recently been adopted in looms with single warp beams, each device including an electric motor for rotating the beam in a controlled manner and a regulator which controls the speed of the electric motor in dependence on signals provided by an electrical sensor for detecting the tension of the warp threads. The signals provided by the sensor are processed to determine, in particular, the rate at which the beam must rotate in order to keep the tension of the warp as uniform as possible. The actual rate of rotation of the beam is detected, for example, by a tachometric dynamo connected to the motor which drives the beam and is compared with the speed calculated on the basis of the signals provided by the warp-tension sensor. The regulator then corrects the speed-of the-beam, bringing the actual speed of the beam to the value calculated on the basis of the warp tension detected.
Such a warp-tension regulator is constituted, for example, by the patented device produced and marketed by the Applicant under the trade name .cent.TENDILENE".
With reference again to the problem of controlling the unwinding of the warp in a loom with twin beams, it has been proposed to provide the twin beams with respective devices of the type mentioned above for controlling the unwinding of the warp. This solution, in which the two devices for controlling the unwinding of the warp operate independently, represents very considerable progress compared with the solution based on the use of mechanical differential devices.
There are some situations, however, in which the use of two separate and independent devices for controlling the unwinding of the warp does not prevent the formation of defects in the fabric which may arise even as a result of extremely small variations in the lengths of the yarns unwound from the two beams according to their tension.